The rapid evolution of Vehicular Ad Hoc Networks (VANETs) demands authentication mechanisms that are not only secure, but also agile enough to handle the diverse, high-mobility interactions of intelligent transportation systems. This paper introduces a novel lightweight and scalable authentication protocol that unifies security across all major VANET communication modes Vehicle-to-EdgeRSU,Vehicle-to-Vehicle, EdgeRSU-to-EdgeRSU, EdgeRSU-to-IoV Controller, IoV Controller-to-Distributed Server and User-to-Distributed Server. Our approach integrates elliptic-curve cryptography, hash-based commitments, and ephemeral key exchange to provide strong mutual authentication and end-to-end confidentiality, all while maintaining a minimal computational overhead. Extensive formal and informal security assessments, coupled with performance benchmarking, demonstrate the protocol's strong resistance to prevalent threats and its efficiency for deployment in dynamic, real-time vehicular environments.